At the time, there was little immediate sense that something utterly extraordinary had happened, or that life had changed for ever. After August 6 1945, popular newspapers wrote half nervously and half exultantly about the coming of the "atomic age", but the most widespread reaction was mere thankfulness that the war was over.

It was argued then, and still sometimes is, that the bombing of Hiroshima 60 years ago tomorrow, and of Nagasaki three days later, was justified by the Japanese surrender, obviating the need for an invasion of Japan which would have meant huge casualties. That may not even be true, though the debate among military historians remains unresolved.

By the summer of 1945, Japan was already prostrate. Not only were Japanese armies being driven out of the Pacific islands and Burma, American bombers were wrecking the cities of Japan and, in one of the most successful campaigns of the whole war, submarines of the US navy had done to Japan what German U-boats had never managed to do to England, by completely destroying its shipping. Some American admirals believed then and ever after that surrender was a matter of time, and not much of it, and a strong suspicion persists of an ulterior motive by Washington, wanting to end the war with Japan quickly before Soviet Russia joined in.

In any case, that argument begs the profoundest questions of ends and means. In the shadow of the mushroom cloud, few people addressed them, or grasped the enormity of what had been done. Two who did were very remarkable men writing from entirely disparate perspectives: Dwight Macdonald, an American radical atheist, and Monsignor Ronald Knox, a conservative English Catholic.

Once an active Trotskyist, Macdonald was evolving from revolutionary socialism to pacifist anarchism, as reflected in Politics, the brilliant magazine he published from 1944 to 1949. His response to the news from Hiroshima was unequivocal. "This atrocious action places 'us', the defenders of civilisation, on a moral level with 'them', the beasts of Maidanek. And 'we', the American people, are just as much and as little responsible for this horror as 'they', the German people."

After the two cities were destroyed, Knox was about to propose a public declaration that the weapon would not be used again, when he heard the news of the Japanese surrender. Instead he sat down and wrote God and the Atom, an astonishing book, neglected at the time and since, but as important for sceptics as for Christians.

An outrage had been committed in human and divine terms, Knox thought. Hiroshima was an assault on faith, because the splitting of the atom itself meant "an indeterminate element in the heart of things"; on hope, because "the possibilities of evil are increased by an increase in the possibilities of destruction"; and on charity, because - this answers those who still defend the bombing of Hiroshima - "men fighting for a good case have taken, at one particular moment of decision, the easier, not the nobler path".

That was finely put, by both writers, but there was more to it: should Hiroshima really be seen as uniquely wicked or cataclysmic? However horrific, it may be that it was not so very different in degree, or even in kind, from what had gone before.

In 1939 the British government had entered the war with high protestations of virtue. Neville Chamberlain told parliament: "Whatever be the lengths to which others may go, His Majesty's government will never resort to the deliberate attack on women and children, and on other civilians for the purposes of mere terrorism." By the end of the war, the British had resorted to enough "mere terrorism" to destroy most of the cities of Germany and many of their inhabitants, 100,000 of them children.

This grew out of the exigencies of war and was one of those changes that take place without anyone's really reflecting, or even noticing. And yet it was an immense development. If you had told any Englishman a hundred years ago - not only a pacifist but an army officer - that before the century was out warfare would largely consist of killing civilians, he would have thought you were insane.

But that was what happened. During the recent Kosovo "war", a French officer asked bitterly if this was to be the first war in history in which only civilians were killed, and yet we had long since begun to go down just that road. It is sobering to compare the 300,000 British uniformed servicemen who died in 1939-45 with the 600,000 German civilians killed.

Making war on civilians took a further turn in the Far East, and not only because of the Japanese army's own atrocities towards conquered peoples. Before August 1945, very many Japanese had already been killed by "conventional" bombing. On one night in Tokyo in March, American bombers killed 85,000 civilians - more than would die at Nagasaki - and at least 300,000 were incinerated in great fire raids over the following months.

And so it was that, as Evelyn Waugh put it when writing about Knox's book in 1948: "To the practical warrior the atom bomb presented no particular moral or spiritual problem. We were engaged in destroying the enemy, civilians and combatants alike. We always assumed that destruction was roughly proportionate to the labour and material expended. Whether it was more convenient to destroy a city with one bomb or a hundred thousand depended on the relative costs of production." Hiroshima was but one more step.

However noble Macdonald and Knox's may now seem, it is only fair to point out that one was a conscientious objector living in New York and the other a priest living in a country house in Shropshire.

Their consciences might not have been so acute if they had been in uniform, fighting or about to fight against Japan. To put it in personal family terms, apart from one uncle I never knew who had been killed in Bomber Command (and for all Macdonald's rhetorical flourish, I don't think that he, or even the crew of Enola Gay, were war criminals to be compared to death camp guards); two other uncles had recently been released from German prison camps; and my father, a Fleet Air Arm pilot, was training a new squadron destined for the Japanese war where he had already served. I have never asked any of them, but I imagine that their immediate reaction to the news that August was pure relief. I imagine mine would have been in their place.

Where Macdonald was surely right was to say that nuclear weapons - or what President Harry Truman called "the greatest achievement of organised science in history" - had rendered obsolete the very concept of material, scientific "progress". As the great and heroic Simone Weil had said before her death two years earlier, the evil in modern war was now the technical aspect itself rather than political factors. Everything that has happened since has only confirmed that truth.

· Geoffrey Wheatcroft's most recent book is The Strange Death of Tory England